In HER 39 years, Claudia Hallblom has, by her own estimation, lost and regained about 1,000 pounds.

Her success at losing weight was always driven by a goal, such as looking nice for her graduation or wedding. Her tactics usually included strict calorie-counting. But success on the scales was always fleeting. Sooner or later, she would revert to her old habits and no longer feel motivated to change.

"I didn't know how to lose weight and keep it off," the Downey woman says.

Most people can lose weight. But few can maintain their new weight for long. Researchers are now tackling that problem, and what they're learning is disconcerting. The human body, it seems, is designed to sabotage weight loss at every turn -- once a body has been fatter, it wants to get back to the weight that it used to be. Physiology is cruelly changed in two ways: The body needs fewer calories to maintain itself, but its craving for food is more intense.

Becoming overweight, in other words, is like being issued a credit card with an uncomfortably high balance that you'll probably end up paying off forever. Making sure the pounds stay off means pitting one's willpower against a swarm of biological processes involving the brain, hormones, metabolism and fat storage.

"There is a big shift toward understanding long-term weight maintenance," says Paul MacLean, associate professor of medicine at the University of Colorado, Denver. "We have a huge number of diet books and diet programs, and if you do them, you can lose weight. The big problem is keeping it off. The recent estimates are that 5% to 10% of people are successful at keeping weight off on a long-term basis."

But before you throw up your hands and reach for the Twinkies, consider this: Scientists think the truth will set us free -- that understanding the stubborn biological processes at work will lead to ways to fight back and outsmart them.

Exercise, it's known, buffers the post-diet body against regaining weight, in ways that researchers are just starting to comprehend. Certain foods, scientists believe, may help stave off weight regain too. And medications now in development target some of the biochemistry thought to be linked to packing the pounds back on.

"There are strong physiological adaptations to weight loss that promote weight regain," MacLean says. "The good news is we know a big part of the problem and why we haven't been successful over the past several decades."

The energy gap

Human biology -- for obvious adaptive reasons -- is designed to protect against weight loss and potential starvation. And after a period of obesity, the body may permanently alter the way weight is regulated by more aggressively stimulating appetite and signaling the body to protect fat stores.

Metabolism has changed: the body now needs about eight fewer calories per day for each pound of weight that was lost. That means someone who loses 40 pounds will require about 320 calories fewer each day than they did before the weight loss. This difference in energy needs before and after weight loss has been dubbed the "energy gap" by University of Colorado professor James O. Hill, director of the Center for Human Nutrition in Denver.

Appetite hormones change too. The hormone leptin, for example, is a major appetite regulator -- it tells the body to stop eating and store fat after meals. Some people may be genetically prone to having lower leptin levels, making them more prone to obesity. But studies also show that, after a weight loss, leptin levels are lower than what they used to be. That means appetite is less easily quelled. It's like a car that has suddenly lost its brakes.

Another hormone, ghrelin, stimulates food intake -- levels in the brain fall lower after a meal. However, after a weight loss, ghrelin levels in the blood generally increase, and the fall-off after mealtimes isn't as marked.

"You lose 10% of your body weight. All of a sudden all these systems kick in to try to keep you from losing weight," says Dr. Ken Fujioka, director of nutrition and metabolic research at the Scripps Clinic in San Diego. "People are mad at themselves or depressed after they regain the weight. But I explain: It's not you. Biology has kicked in now. . . . You are hungry all the time. You think about food all the time."

The brain isn't the only thing acting to promote weight regain. MacLean's research suggests that the central nervous system collects and interprets signals from the intestines and peripheral tissues, such as fat stores in the abdomen, to fight weight loss or regain pounds that were lost.

Through this sensing, when calories consumed surpass calories expended, the body alters how it uses and stores fuel. Glucose becomes the preferred fuel for energy, and fat is directed to fatty tissue stores in the body. Excess glucose is also converted to fat. And studies performed at the University of Toronto using continuous glucose monitoring devices show the blood sugar levels of obese people spike and plummet routinely throughout the day while normal-weight people have more stable levels.

Each drop is a cue to eat, says Dr. Michael R. Lyon, medical and research director for the Canadian Center for Functional Medicine.

The weight comes back fast. "The entire system is saying, 'Bring the calories in, store them efficiently and do not reduce these signals until the fat is returned that was there before,' " MacLean says. "You may look like a lean person, but your body hasn't changed inside."

Moreover, animal studies show that most of the regained weight is distributed as visceral fat, the abdominal paunch that is linked to heart disease and diabetes.

Tough, but losable

So what is a dieter to do?

"There is nothing we know of that does anything to reverse this," Fujioka says of the biological forces that defend body fat. "It's very tough."

But it's not impossible. The National Weight Control Registry is an honor roll of dieters who have fought and won. Started in 1994 with modest expectations by Hill and Rena Wing, director of the weight control and diabetes research center at Brown Medical School, the registry now provides some cherished data on how regular people have managed to keep weight off. The registrants, who are surveyed regularly, have maintained a weight loss of at least 30 pounds for at least one year.

Based on data from more than 7,000 people, Wing says there are few similarities in how people lose weight. But those who succeed in maintenance sing the same song.

Instead of trying to eat less for the rest of their lives to bridge the energy gap, these people exercise more. They typically spend an hour or more each day in aerobic exercise and strictly limit time spent watching television.

Physical activity, in ways that researchers don't really understand, influences some of the biological systems that promote weight regain, encouraging the body to become more sensitive to leptin and insulin, for example.

"Everyone thinks exercise is about burning calories," Fujioka says. "But you are actually returning the system to more like what it should be. Things start working again."

The successful maintainers also change what they eat: The registry found that they keep their calories in careful balance with what they expend -- religiously referring to calorie charts and writing down everything they consume. They also tend to eat low-fat foods.

But there may be more nuances to food choices than that. "We're getting more interested in studies that look at food composition," Fujioka says. "It could be that eating certain nutrients may also help the system work better."

Studies suggest that calcium, for example, may help people regulate their weight, he says. No one really understands how calcium may do this -- in fact, the theory is controversial. But it could be that a diet high in calcium suppresses a form of vitamin D called calcitriol that revs up fat-burning processes.

Other research has focused on foods that balance blood-sugar levels, such as low-glycemic and fibrous foods. Studies show that eating low-glycemic foods, such as lentils and nuts and foods with high water or fiber content, helps stabilize blood sugar and curb the brain signals that urge people to eat. Fiber does this by slowing the absorption of carbohydrates from food, which helps lower their glycemic load.

"Weight loss alone is not a realistic goal," Lyon says. "It can do more harm than good. The key is to get your brain back on your side. The starting point of that is stabilizing blood sugar."

It's easier after two years

Scientists don't know how long it would take to return the physiological responses of a once-obese body to normal -- or if, indeed, that ever is quite possible.

Studies do show, however, that weight regain is most likely in the first couple of years after weight loss. And Wing says that registry data shows that people who maintain their weight loss find the first two years difficult but eventually adapt comfortably to their new habits and lifestyles.

"After that, it's as if you master the technique," she says.

The current research on obesity strongly points to two messages that rely heavily on human behavior: Don't gain excess weight in the first place, and if you do, be prepared to make permanent lifestyle changes to lose it and maintain the loss.

Thanks for posting this. Truer words were never spoken. I've been maintaining for a year and a half, and it's harder than losing. It's also not as rewarding...yes, glad to see the scale stay the same, but that's nowhere near as thrilling as a drop. Sometimes I wonder if I unconsciously sabotage myself so I can go back to losing mode, and sometimes it honest to goodness feels like my body is craving more and more calories trying to get itself back "up."

I also believe that knowing your enemy is valuable, so the more I can read about this and the more things I can put into practice to win this fight, the better.

__________________Cassie

And now that you don't have to be perfect, you can be good.
-John Steinbeck

Thanks for posting this. Truer words were never spoken. I've been maintaining for a year and a half, and it's harder than losing. It's also not as rewarding...yes, glad to see the scale stay the same, but that's nowhere near as thrilling as a drop. Sometimes I wonder if I unconsciously sabotage myself so I can go back to losing mode, and sometimes it honest to goodness feels like my body is craving more and more calories trying to get itself back "up."

I also believe that knowing your enemy is valuable, so the more I can read about this and the more things I can put into practice to win this fight, the better.

Talk about "truer words never spoken".

I can see myself being in exactly the same predicament soon. I've actually reached my goal but decided to drop it another 5 (slight belly fat left). I'm really nervous about maintenance and have started thinking about it. Basically the strategy is to maintain eating clean, keep exercising, but allow for a a gradual increase in more calories per day while keeping an eye on the scale until there's a leveling off. And of course tracking everything on FitDay.

Couldn't agree more about the subconscious sabotaging as I've been there before. But not this time is the promise to myself, and now I've got the tools and the resources to make good on this promise.

Great article! I've been there, done that, (twice) and have the 8x T Shirts to prove it. Well this time, to be perfectly blunt, $%#@ the million-odd years of evolution that say I will be super obese again. It's just NOT going to happen.

I decided long ago that I must track my calories forever, and I can live with that. I'm glad that the researchers found that people who do that have maintained their losses. I think I'm old and wise enough now to know my own limitations, to accept them, and to work out a plan that I can live with and still maintain a healthy weight level.

Very interesting but this confuses me:
"Studies suggest that calcium, for example, may help people regulate their weight, he says. No one really understands how calcium may do this -- in fact, the theory is controversial. But it could be that a diet high in calcium suppresses a form of vitamin D called calcitriol that revs up fat-burning processes."

So they are saying that calcium is good for weight loss right (or maintaining as opposed to regaining)? If so, then why does it suppress something that "revs up fat-burning processes."?

Great article! I've been there, done that, (twice) and have the 8x T Shirts to prove it. Well this time, to be perfectly blunt, $%#@ the million-odd years of evolution that say I will be super obese again. It's just NOT going to happen.

I decided long ago that I must track my calories forever, and I can live with that. I'm glad that the researchers found that people who do that have maintained their losses. I think I'm old and wise enough now to know my own limitations, to accept them, and to work out a plan that I can live with and still maintain a healthy weight level.